The present invention relates to an improved process for producing pure carboxylic acids by catalytic liquid phase oxidation of a suitable precursor in a solvent, and, more particularly, to a process for producing highly pure terephthalic acid according to such process by conducting the oxidation reaction in a plug flow reaction zone at a high solvent:precursor ratio, temperature and pressure sufficient to maintain the terephthalic acid in solution as it is formed. Thereafter, pure terephthalic acid is systematically crystallized from the resulting reaction medium and recovered as pure crystals without the need for separate purification.
Pure terephthalic acid, an important raw material used in the production of poly(ethylene terephthalate), i.e., PET, for conversion into fibers, films and containers, is commercially produced by purifying crude- or technical-grade terephthalic acid. Practically all technical-grade terephthalic acid is produced by catalytic, liquid phase air oxidation of paraxylene. Commercial processes use acetic acid as a solvent and a multivalent heavy metal or metals as catalyst. Cobalt and manganese are the most widely used heavy metal catalysts, and bromine is used as a renewable source of free radicals in the process.
Acetic acid, air (molecular oxygen), paraxylene and catalyst are fed continuously into an oxidation reactor that is maintained at from 175.degree. C. to 225.degree. C. and 1500-3000 kPa (i.e., 15-30 atm). The feed acetic acid:paraxylene ratio is typically less than 5:1. Air is added in amounts in excess of stoichiometric requirements to minimize formation of by-products. The oxidation reaction is exothermic, and heat is typically removed by allowing the acetic acid solvent to boil. The corresponding vapor is condensed and most of the condensate is refluxed to the reactor. Two moles of water are formed per mole of paraxylene reacted, and the residence time is typically 30 minutes to 2 hours, depending on the process.
The effluent from the reactor is a slurry of crude terephthalic acid crystals which are recovered by filtration, washed, dried and conveyed to storage. They are thereafter fed to a separate purification step. The main impurity is 4-carboxybenzaldehyde (4-CBA), which is incompletely oxidized paraxylene. Although the purity of crude-grade terephthalic acid is typically greater than 99%, it is not pure enough for the PET made from it to reach the required degree of polymerization.